1. Field of the Invention
The present invention relates to optical elements, optical heads and optical recording/reproducing apparatuses using the same, and optical recording/reproducing methods.
2. Description of the Related Art
Digital versatile disks (DVDs) can record digital information at a recording density that is about six times as high as that of compact disks (CDs), so that they are noted as high-capacity optical recording media. In order to reproduce high-density DVDs, the wavelength of the laser beam has to be shorter, and the numerical aperture (NA) of the objective lens has to be larger than for reproducing CDs. Therefore, a laser beam with a wavelength of 650 nm is used for reproducing DVDs (compared to 780 nm for CDs), and an objective lens with a NA of 0.6 is used (compared to 0.45 for CDs). Attempts have been made to increase the recording density by making the wavelength of the laser beam even shorter and the NA even larger. However, when the wavelength is shortened and the NA of the objective lens is increased, the recording and reproducing margins for positional deviations of the recording layer in the thickness direction become small. Consequently, in that case, it is necessary to perform a correction of spherical aberration.
Moreover, optical recording media provided with a plurality of recording layers have a high recording density, but in that case, the distance from the surface of the optical recording medium to the recording layer (also referred to as “base material thickness” in the following) varies from recording layer to recording layer, thus causing spherical aberration. To correct this spherical aberration, an optical head correcting wavefront aberration (in particular, spherical aberration) with a liquid crystal element has been proposed (JP H10-269611A).
An example of this conventional optical head is explained with reference to FIG. 13. FIG. 13 schematically illustrates the configuration of a conventional optical head 200 (also referred to as an “optical pickup”). As shown in FIG. 13, the optical head 200 includes a light source 201, a polarizing beam splitter 202, a liquid crystal panel 203, a λ/4 plate 204, an objective lens 205, a focusing lens 206, an optical detector 207, a base material thickness sensor 208, and an optical element driving circuit 209. Signals are recorded on or reproduced from the optical disk 210 with this optical head 200.
The light source 201 is made of a semiconductor laser element, which emits coherent light for recording/reproducing towards the recording layer of the optical disk 210. The polarizing beam splitter 202 serves as an element for separating the light. The liquid crystal panel 203 includes a plurality of electrodes 203a to 203d, arranged concentrically as shown in FIG. 14, which change the refractive index of the liquid crystal by applying different voltages to the electrodes, thus correcting aberration. The λ/4 plate 204 is made of a birefringent material, and converts linearly polarized light into circularly polarized light. The objective lens 205 focuses light on the recording layer of the optical disk 210. The focusing lens 206 focuses light that has been reflected by the recording layer of the optical disk 210 on the optical detector 207. The optical detector 207 receives the light that has been reflected by the recording layer of the optical disk 210 and converts it into an electrical signal.
The following is an explanation of the operation of this optical head. Linearly polarized light that is emitted from the light source 201 passes through the polarizing beam splitter 202 and enters the liquid crystal panel 203. If the recording layer of the optical disk 210 is arranged at a position that is different from the design value, the base material thickness sensor 208 detects this deviation, and outputs this deviation to the optical element driving circuit 209. Based on the received deviation, the optical element driving circuit 209 drives the liquid crystal panel 203 to correct the wavefront aberration caused by this deviation. Consequently, a wavefront aberration that corrects the wavefront aberration caused by the deviation of the base material thickness (third-order spherical aberration) is imparted on the light entering the liquid crystal panel 203.
The following is a more detailed explanation of a method for correcting spherical aberration with the liquid crystal panel 203. First, FIG. 15 shows the phase distribution when the base material thickness of the optical disk 210 deviates from the design value (i.e. the optimum base material thickness). FIG. 15 shows the phase distribution for a laser beam wavelength of 405 nm, an NA of the objective lens of 0.85, an optimum base material thickness of the optical disk 210 of 0.1 mm, and a base material thickness deviation of 0.01 mm, illustrating the distribution of the wavefront aberration on the recording layer of the optical disk 210 at the best image point. If a phase correcting this distribution completely is added to the laser beam, then the spot of the laser beam on the optical disk 210 can be constricted to the diffraction limit, even though the base material thickness of the optical disk 210 deviates from the optimum base material thickness.
In order to correct the wavefront aberration in FIG. 15, a phase change canceling the wavefront aberration in FIG. 15 should be imparted on the laser beam. That is to say, the optical path length should be partially changed. With a liquid crystal, the optical path length can be partially changed by changing the voltage applied to the liquid crystal, because its refractive index depends on the voltage applied to it. Consequently, the spherical aberration shown in FIG. 15 can be corrected by applying a suitable voltage to the electrodes 203a to 203d shown in FIG. 14.
However, in the optical head 200, spherical aberration is corrected by generating third-order spherical aberration, so that the effect of correcting spherical aberration is poor when the center of the objective lens 205 deviates from the center of the electrodes 203a to 203d of the liquid crystal panel 203. That is to say, if the objective lens 205 and the liquid crystal panel 203 are arranged in separation from each other, then the center of the objective lens 205 deviates from the center of the electrodes 203a to 203d of the liquid crystal panel 203, due to the shifting of the objective lens 205 in accordance with the eccentricity of the optical disk 210, thus worsening the corrective effect.
FIG. 16 illustrates the relation between the deviation of the center of the objective lens 205 from the center of the electrodes 203a to 203d and the aberration after the correction, when the wavelength of the laser beam is 400 nm, the NA is 0.85, and the base material thickness of the optical disk 210 deviates 10 μm from the design value (0.1 mm).
As shown in FIG. 16, when the center of the objective lens 205 deviates from the center of the electrodes 203a to 203d, the corrective effect deteriorates. This is because, due to the deviation between the centers, the spherical aberration generated by the liquid crystal panel 203 causes coma aberration. In order to prevent deviation of the centers, it is necessary to form the liquid crystal panel 203 in one piece with the objective lens 205.
However, when the liquid crystal panel 203 is formed in one piece with the objective lens 205, it is difficult to make the optical head thinner. Also, since it becomes necessary to shift the liquid crystal panel 203 together with the objective lens 205, the frequency response (sensitivity) of the actuator drops. Moreover, the wiring to drive the liquid crystal panel 203 makes manufacture of the actuator more complex, so that it becomes difficult to lower the costs.
As a method for correcting spherical aberration, it has been proposed to correct spherical aberrations by arranging two lenses on the optical axis and changing the spacing of the lenses (see JP 2000-131603A). In this method, changing the spacing of the lenses imparts a phase change on the light passing through the lenses, which changes parallel light into divergent or convergent light, thus correcting spherical aberration. However, this method necessitates a mechanical means for changing the spacing of the lenses in accordance with the deviation of the base material thickness, which makes miniaturization of the optical head difficult. Moreover, to prevent the occurrence of coma aberration, the centers of the two lenses have to be matched precisely, which makes it difficult to manufacture the optical head at low cost. Furthermore, in this method, the spacing of the lenses is changed on the optical axis, so that the optical system becomes magnifying or contracting. As a result, there is the problem that the transmission efficiency of light that is incident on the lenses for correcting spherical aberration changes, and the rim intensity of the light changes.
The present invention has been developed in view of these problems, and it is a first object of the present invention to provide an optical element with which an optical head can be configured, in which there is little deterioration of the correctional effect when the objective lens shifts, as well as an optical head and an optical recording/reproducing apparatus using such an optical element. It is a second object of the present invention to present a novel optical recording/reproducing apparatus and optical recording/reproducing method.